cmake: Rename installhdrs to install-llvm-headers and fix the dependencies

The installhdrs target was inconsistently named and would behave
differently depending on whether or not you ran a build first. This
renames it to install-llvm-headers to match other target names and
adds a dependency on intrinsics_gen so that it will always install the
same set of things.

llvm-svn: 285035
1 file changed